1. Technical Field
The present invention relates to a graded index optical device which can particularly be applied to an optical device such as an optically coupled component for optical communication, a camera, an endoscope or the like, and a method for manufacturing the graded index optical device by a sol-gel process.
2. Description of Related Art
A graded index optical device has received attention, for example, as a lens having a small spherical aberration used when radiant light from a semiconductor laser is made highly efficiently incident on an optical fiber, and also, as a lens for image transmission of a camera or the like since an edge face of the lens is smooth and an optional focal length can be provided by changing the length of a circular cylinder. In particular, the shape is compact and cylindrical, thus it has been employed as a lens that is easily aligned with an axis of an optical system, easily retained and has high assembly performance. As shown in FIG. 1, this cylindrical graded index optical device (Graded Index lens, hereinafter abbreviated as “GRIN lens”) is a rod lens which continuously changes by a square curve when a refractive index n in cross sectional directions x and y is expressed by the following equation (1), and the function of the lens is performed by this refractive index distribution:n=n0(1−g2r2/2)  (1)where g is a constant representing light-condensing performance of the GRIN lens, n0 is a refractive index of a GRIN lens material, and r is a radial direction given by r2=x2+y2.
As shown in FIG. 1, when a is a radius of the GRIN lens and na is a refractive index at a radius a, the constant g is expressed by g=NA/an0.
Note that a NA is expressed by NA=(n02−na2)1/2 (2).
In this equation, the NA is the square root of a difference of two squares in the center and on the periphery of the GRIN lens, is referred to as a Numerical Aperture (hereinafter referred to as “NA”), and is an important parameter representing lens performance. A lens with a high NA is a lens having high light-condensing performance, and in other words, the lens characteristics are excellent. At present, a GRIN lens which has become commercially practical has a NA of about 0.2. Therefore, there is a strong desire to provide a GRIN lens that has a NA≧0.4 for making the radiant light from the semiconductor laser highly efficiently incident on the optical fiber, and also has a small diameter for miniaturizing an optical device.
As a method for making the GRIN lens, an ion exchange process, a vapor phase CVD (Chemical Vapor deposition) process, a sol-gel process or the like is known. However, a GRIN lens made by the ion exchange process is multicomponent glass containing an alkali component. Hence, this GRIN lens can not provide a large NA, and further is poor in reliability because of a problem of heat resistance due to a very large coefficient of thermal expansion of a glass material. Further, the vapor phase process obtains a NA of 0.38 on an experimental level (see for example, P. B. O'Connor et al., Electron Lett., 13, (1977) 170-171). However, in order to obtain a NA of 0.38 or more, it is necessary to increase loads of additives (GeO2, P2O5 or the like). As a result of this, the coefficient of thermal expansion of the glass material becomes large, a base material tends to easily crack and air bubbles are easily generated during vitrification, thus this process is too unstable for practical use. Among others, a method for making glass by the sol-gel process that is capable of accurately forming a desired concentration distribution is effective.
On the other hand, since the sol-gel process is a synthesis method performed at a low temperature, the concentration distribution of a refractive index distribution imparting metal can precisely be formed, thus it is an effective process. In the method for making glass by the sol-gel process, an alcoholic solution including an alkoxide of silicon as a main component is added to acids or bases as solvents and hydrolysis is performed to make sol. When the multicomponent glass is made, a metal component is further added and this sol is further subjected to a polycondensation reaction, thus a crosslinking reaction proceeds to make a wet gel. Then, the obtained wet gel is dried, the solvent in the gel is removed, and thereafter, the gel is sintered to thereby make dense glass.
When the GRIN lens is made using the sol-gel process, it is necessary to form the concentration distribution on a refractive index distribution imparting a metal component. As other such methods, there is known a method using a metal salt as a raw material of a metal component, a molecular stuffing method, and further a method using a metal alkoxide. In a method for introducing the metal component using metal salt to form a refractive index distribution, gel added with the metal salt as an aqueous solution or an alcoholic solution during preparation of sol is prepared. The gel is present in a state in which the metal salt is dissolved in a solvent in a pore formed of a skeleton of silicon. The obtained gel is immersed in alcohol with a low molecular weight, water, a mixed solution thereof or the like having high solubility to the metal salt, thereby eluting the metal component contained in the gel to form the concentration distribution. However, since an appropriate salt of a metal such as Ti, Nb, Ta, Zr or the like which extensively contributes to a refractive index is not present, it was significantly difficult to apply this process to form a GRIN lens having a high NA.
In the molecular stuffing method, Japanese Examined Patent Publication No. H5-82332 discloses a method in which a wet gel is dried and sintered to make a porous body, and the porous body is immersed in a Ti containing solution or the like to uniformly impregnate it with a metal component, thus metal containing glass such as SiO2—TiO2 or the like is obtained. However, in this method, since a dry gel is thermally treated with a high temperature, a bonding hand of Si—O—Si is securely bonded and a reactive Si—OH group becomes few in number, thus only a very few metal components can bond to Si—O—Si, therefore it was difficult to stably make a GRIN lens with a high NA.
In the method using the metal alkoxide, gel is made in which the metal alkoxide as a component for enhancing a refractive index is added to an alkoxide of silicon as an alcoholic solution during preparation of sol. Since the gel using the metal alkoxide forms a bond between silicon and a metallic atom, in order to form a concentration distribution on a metal component, this method cleaves the bond between silicon and the metallic atom, immerses the gel in a concentration distribution imparting solution capable of eluting the metal component, washes an eluted liquid, then fixes the concentration distribution, and carries out washing, drying and sintering. In the method using this metal alkoxide, in particular, when SiO2—Ta2O5 or SiO2—TiO2 quartz glass is selected, a GRIN lens having a high NA can stably be produced. However, to manufacture the GRIN lens, it is necessary to perform each process of eluting the metal component in the gel, washing, drying and sintering while carrying a porous gel that is easily cracked by mechanical shock and is also easily shrunken by applying heat. In particular, it is significantly difficult to accurately and stably manufacture a GRIN lens having a small diameter of 1 mm or smaller without cracking, and thus it has been impossible to commercially mass-manufacture. Conventionally, this method has been capable of mass-manufacturing a diameter of about 10 mm at a wet gel stage, a diameter of about 5 mm at a dry gel stage, and a diameter of about 2 to 3 mm at a stage sintered into the GRIN lens.
Patent Document 1: Japanese Examined Patent Publication No. H5-82332
Non-patent Document 1: P. B. O'Connor et al., Electron Lett., 13, (1977) 170-171